Absorbent polymer material based on renewable starting materials

ABSTRACT

A method of production of a highly absorbent, polysaccharide-based material, wherein an aqueous solution containing a starting material including a cross-linkable polysaccharide-based polymer blend of an electrically charged polysaccharide-based polymer and an electrically uncharged polysaccharide-based polymer is subjected to cross-linking in order to obtain a water-swelled gel. The cross-linked, water-swelled gel is dessicated with a polar solvent.

TECHNICAL FIELD

The invention pertains to a method for manufacturing of a highlyabsorbent, polysaccharide based absorption material, wherein awater-containing solution comprising a starting material in the form ofa crosslinkable polysaccharide based polymer is subjected tocrosslinking in order to obtain a water-swelled gel.

BACKGROUND OF THE INVENTION

For many applications, such as in absorbent articles intended forabsorption of body fluids, it has become increasingly more common to usewhat is known as superabsorbent materials. Superabsorbent materials arepolymers which are capable of absorbing liquid in amounts correspondingto several times of the weight of the polymer and which upon absorptionform a water-containing gel.

The main advantage of using superabsorbent materials in absorbentarticles is that the volume of the absorbent articles can beconsiderably reduced when compared to the volume of absorbent articlesmainly formed from absorbent fibrous materials such as fluffed cellulosepulp, or the like. Another advantage is that superabsorbents, whencompared to fibrous absorbents such as, for instance, fluffed cellulosepulp, have a higher capability of retaining liquid under pressure. Sucha property is, for instance, advantageous when the absorption materialis used in diapers, incontinence guards or sanitary napkins, sinceabsorbed body fluid is retained in the absorbent article and is notsqueezed out of the article, for instance when the user is sitting.

However, a disadvantage with many of the superabsorbent materialspresently being used in absorbent articles such as diapers, incontinenceprotectors or sanitary napkins, is that they are not produced fromrenewable raw materials. In order to solve this problem, it has beensuggested that superabsorbents based on different types of renewablestarting materials, such as polysaccharides and, in particular, starch,be used. Unfortunately, the polysaccharide-based superabsorbents whichhave so far been produced exhibit considerably lower absorption capacitythan the commonly used polyacry-late-based superabsorbents. Further, theability of the polysaccharide-based superabsorbents to retain absorbedliquid when the superabsorbent is subjected to load is low in comparisonwith polyacrylate-based superabsorbents.

In WO 95/31500 a method for producing absorbent, preferablysuperabsorbent, foam materials by phase separation and crosslinking of apolymer solution is described. The absorbent materials thus obtainedexist in the form of a crosslinked open-celled polymer foam, whichimplies that fluid may pass between cells. By means of the describedproduction method, it is also said to be possible to obtainbiodegradeable absorbent foam materials. Preferred polymers forproducing the absorbent materials which are disclosed in WO 95/31500 arehydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), whichare preferably crosslinked with divinyl sulphone (DVS).

The known absorbent foam materials are relatively expensive to produceand are primarily intended for medical applications, such as controlledrelease systems or as artificial skin and blood vessels. However, afurther possible use for the described foam materials is said to be inreusable diapers or the like. The high production cost does, however,mean that the known foam materials would, in practice, not becontemplated as absorption material for absorbent articles intended forsingle use only.

For these reasons, there exists a demand for an improved superabsorbentmaterial based on renewable raw materials. Accordingly, the absorptioncapacity for polysaccharide-based superabsorbents needs to be improvedin order to make such superabsorbents an equal alternative with regardto absorbency and when compared to the superabsorbents which arecommonly being used today. Moreover, there exists a need for asuperabsorbent material for use in disposable absorbent articles andwhich is produced from cheap and readily available renewable startingmaterials.

SUMMARY OF THE INVENTION

The present invention provides a process for the production ofsuperabsorbent materials of the kind mentioned in the introduction andwhich exhibit improved absorbency as compared to previously knownsuperabsorbent materials of the same type.

The process according to the invention is primarily distinguished inthat drying of a crosslinked liquid-swollen gel is carried out byextraction with a polar solvent.

A wide range of solvents may be used for the initial solution,containing the polysaccharide-based polymer starting material. However,the solution containing the starting material is preferably an aqueoussolution.

Surprisingly, it has been shown that by drying a crosslinkedpolysaccharide with a polar solvent, such as ethanol, acetone orisopropanol, a superabsorbent material can be obtained exhibitingsuperior absorbency when compared to a material of the same compositionbut dried using another method. The improved absorbency is evident bothin a higher absorption capacity and in a greater ability to retainabsorbed liquid even when the absorption material is subjected topressure. The absorbency of a superabsorbent material which has beendried with a polar solvent is considerably higher than that of acorresponding superabsorbent material which has been dried using anyother method, regardless of whether the absorbed liquid is water or asalt solution such as urine.

When comparing electron scanning micrographs of crosslinkedsuperabsorbent gels with the same composition but dried in differentways, it is clearly evident that the microstructure of the dried gels,or xero-gels, show significant differences depending on the method ofdesiccation. Accordingly, an air-dried gel exhibits a dense, compactstructure while a gel which has been dried by solvent extractionexhibits a structure with a high degree of microporosity. Vacuum dryingproduces a structure exhibiting some degree of microporosity and can besaid to represent a form between the structure obtained by air-dryingand the structure obtained by the solvent drying in accordance with theinvention.

A probable explanation of the advantageous effect of solvent drying, isthat a commonly occurring phenomenon producing a dense, horny,non-absorbing structure, is avoided. This phenomenon is well known tothe person skilled in the art, even though its exact mechanisms have notyet been fully understood. However, the effect is that the crosslinkedgel exhibits reduced swelling capability and, thus, reduced absorptioncapacity. Accordingly, in comparison with conventionally dried gels, agel which has been dried with a polar solvent exhibits a more open andflexible structure, something that affects the absorption process in apositive way.

The solvent-dried superabsorbent polymer exists in the form of amicroporous gel. The superior absorption properties exhibited by the gelare believed to be the result of liquid partly being bound in the gel ina conventional manner and partly being absorbed in the microvoids in thegel. When the gel absorbs liquid, the gel swells, whereby the size ofthe microvoids increases and the absorption capacity of the gel isenhanced.

The starting material may comprise a polymer blend comprising anelectrically charged polysaccharide-based polymer and an electricallyuncharged polysaccharide-based polymer. The ratio between the chargedpolymer and the uncharged polymer is preferably between about 2:1 andabout 4:1 and most preferably about 3:1.

A major advantage afforded by the invention is that carboxymethylcellulose (CMC) can be used as a starting material for the production ofa superabsorbent material having high absorption capacity and goodliquid retention. The fact that CMC is produced from wood which is arenewable material source and, further, that it is readily available andcomparatively low in cost, makes CMC particularly suitable for use indisposable absorbent articles. Moreover, with regard to biodegradabilityand compostability, CMC exhibits excellent characteristics.

However, it has been found to be less suitable to use CMC as solestarting material for the production of a superabsorbent material, sinceCMC tends to form intramolecular crosslinks instead of crosslinksbetween different molecules. An absorption material having particularlygood properties may, however, be obtained with a starting materialcomprising a mixture of CMC in the form of its sodium salt (CMCNa) andhydroxyethyl cellulose (HEC). A suitable proportion between the amountof CMCNa and HEC has thereby been found to be between about 2:1 andabout 4:1 and preferably about 3:1. At a lower concentration of HEC, theresulting cross-linked gel does not exhibit sufficient gel strength.High concentrations of HEC should be avoided since the swelling capacityand, accordingly, the absorption capacity will be insufficient if theHEC concentration is too high.

Alternatively, it is possible to use combinations of other charged anduncharged polysaccharides. Some further examples of suitable chargedpolysaccharides are carboxymethyl starch, oxidized starch and oxidizedcellulose. Suitable uncharged polysaccharides include, but are notlimited to: ethylhydroxyethyl cellulose (EHEC), hydroxypropyl cellulose(HPC) and hydroxypropyl starch (HPS).

It is further possible to use pectin as starting material.

The polysaccharides are preferably crosslinked with a crosslinking agentproducing covalent crosslinks. Some examples of crosslinking agents ofthis kind are divinylsulphone (DVS), acetaldehyde, formaldehyde,glutaraldehyde, diglycidyl ether, diisocyanates, dimethyl urea,epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate,trimethylomelamine, polyacrolein. Naturally, it is also possible to useionic crosslinking or physical crosslinking such ashydrophobic/hydrophilic interactions.

BRIEF DESCRIPTION OF FIGURES

The invention will hereinbelow be described in greater detail, by way ofexample only, and with reference to the Figures shown in the attacheddrawings, wherein:

FIG. 1 shows the water uptake capability for air-dried gels producedwith different amounts of DVS;

FIG. 2 shows the water uptake capability for gels dried using differentmethods and with different addition of DVS;

FIG. 3 shows the water uptake capability for air-dried gels withdifferent concentration of HEC;

FIG. 4 shows the water uptake capability for gels dried using differentmethods and with different concentration of HEC;

FIG. 5 shows the liquid uptake capability in a solution of NaCl in waterfor gels dried using different methods and with-different DVSconcentrations;

FIG. 6 shows the liquid uptake capability in a solution of NaCl in waterfor gels dried using different methods and with different relations inthe mixture of CMCNa/HEC;

FIG. 7 shows the retention of synthetic urine for gels dried usingdifferent methods;

FIG. 8 shows the percentage of liquid which is released uponcentrifugation of hydrogels dried using different methods;

FIG. 9 shows the swelling capacity for a pectin-based absorptionmaterial after drying using different solvents;

FIG. 10 shows the swelling ratio as a function of polymer concentrationof CMC+HEC in the starting reaction solution; and

FIG. 11 shows the swelling ratio as a function of acetone concentrationin the drying step.

Additionally:

FIG. 12 shows an electron scanning micrograph of an air-dried gel;

FIG. 13 shows an electron scanning micrograph of a vacuum-dried gel;

FIG. 14 shows an electron scanning micrograph of a gel dried byextraction with acetone.

DETAILED DESCRIPTION OF METHODS Preparation of Gel

The hydrogels which were used in the following examples were obtained bycrosslinking mixtures of CMCNa and HEC, using DVS as crosslinking agent.The reason for choosing DVS as crosslinking agent is that DVS gives areliable and reproducible crosslinking result. Thus, DVS is well suitedfor the production of crosslinked materials for use in comparative work.However, the invention shall not in any way be regarded as beingrestricted to the use of DVS as crosslinking agent. Accordingly, and asmentioned above, any suitable crosslinking agent or crosslinking methodmay be used.

The crosslinking reaction was performed in an alkaline aqueous solutionwith 0.02 M potassium hydroxide (KOH) at 20° C. CMCNa and HEC weredissolved in distilled water containing the desired amount of DVS. Afterthorough mixing for 24 hours, potassium hydroxide was added, therebystarting the crosslinking reaction. All reactions were performed with areaction solution having an overall polymer concentration equal to 2% byweight.

After 24 hours, the crosslinked hydrogel was soaked in distilled waterin order to reach equilibrium water sorption. The water surrounding thehydrogel was renewed at least three times. Each time, an amount of watercorresponding to 5 times the weight of the hydrogel, measuredimmediately after the crosslinking reaction, was used. The soakingprocedure was terminated after 36-48 hours. Subsequently, the swelledhydrogel was removed from the water and desiccated.

Desiccation Methods

Three different methods of desiccation were used:

i) air drying at atmospheric pressure

ii) drying under vacuum

iii) drying by extraction with a polar solvent

Air drying consisted simply in leaving the swollen hydrogel at roomconditions (25° C. and 50% relative humidity) until completely dry.

Vacuum drying was performed by placing swollen hydrogels in a containerconnected to a vacuum pump and kept at a pressure equal to 0.01 Torr.

Drying by extraction with a solvent consisted in placing water-swollenhydrogels in the solvent at room temperature and with gentle mixing. Thesolvent was replaced two times and the amount of solvent used each timewas approximately twice that of the swollen hydrogel. The reason whyacetone was used in all examples in which the gels were crosslinked withDVS is that, in contrast to the alcohols, acetone will not react withDVS. However, if crosslinking is carried out in an alternative manner,such as enzymatically, polar solvents such as ethanol or isopropanol maybe used.

After desiccation, the dried gels produced by air drying and vacuumdrying were ground in a laboratory grinder. In the solvent dryingprocess, the stirring caused the gel to break into smaller pieces whichwere directly used in Examples 1-3. All subsequent measurements wereperformed on desiccated gel which had been ground or broken up intosmaller pieces.

Free Swelling

Free swelling was determined using two different methods. Accordingly,in Examples 1-3, the ability of the gel to absorb liquid was measuredaccording to a first method by immersing a piece of the gel in the testliquid and allowing the gel to absorb liquid until saturated. The gelwas subsequently removed from the liquid and weighed.

In Example 4, the free swelling capacity on absorption was measuredaccording to a second method by introducing 0.100 g +0.002 gcrosslinked, dried gel in a test tube having the dimensions 150 mm×16mm. The test tube was provided with a screw cap and had a volume of 20ml. The height of the dry, unswollen sample was measured with amillimeter stick and was recorded. Thereafter, 15 ml synthetic urine wasadded with an automatic pipette.

The composition of the synthetic urine (SUR) was 60 mmol/l KCl, 130mmol/l NaCl, 3.5 mmol/l MgSO₄, 2 mmol/l CaSO₄.2H₂O, 300 mmol/l urea, 1g/l of a 0.1% solution of Triton X-100 which is a surfactant sold byAldrich. The sample was left to swell for 2 hours until equilibrium wasreached, whereafter the height was again measured and recorded.

From the thus obtained measurements, the change in volume/weight wascalculated according to:${A\quad (T)} = \frac{( {{h\quad (s)} - {h\quad (t)}} )*\Pi*r^{2}}{m\quad (t)}$

A(T)=Absorption capacity in cm³/g

h(s)=height in millimeters for the swollen sample

h(t)=height in millimeters for the dry sample

m(t)=the dry weight in grams for the sample

r=the radius of the test tube in millimeters (0,72 mm)

DESCRIPTION OF EXAMPLES Example 1

The water uptake capability was measured according to the free swellingmethod, for different gels obtained by crosslinking an aqueous solutioncontaining 2 percent by weight of a mixture of CMCNa and HEC, whereinthe relation CMCNa:HEC=3:1, and with different amounts of crosslinkingagent, divinylsulphone (DVS).

As can be seen in FIG. 1, the swelling capability for a gel-dried underroom conditions (25° C., atmospheric pressure and 50% relative humidity)decreases with increasing content of DVS. The reason for this is that ahigher degree of crosslinking increases the resistance to swelling ofthe gel. At a DVS-content below the lowest content of 0.04 mol/l givenin FIG. 1, the gel strength of the resulting gel is not sufficientlyhigh for the gel to be useful in practice.

FIG. 2 illustrates how different desiccation methods affect the wateruptake capability for the xero-gels presented in FIG. 1. As is clearlyevident from FIG. 2, the gel which has been dried by extraction withacetone has a higher water uptake capability than correspondingair-dried and vacuum-dried gels. This statement is true regardless ofthe DVS-content.

Example 2

The water uptake capability was measured for different gels obtained bycrosslinking and drying of a CMCNa/HEC-mixture in an aqueous solutioncontaining 2 percent by weight of the CMCNa/HEC-mixture and with 0.04mol/l DVS as crosslinking agent and further at different mixing ratiosfor CMCNa:HEC.

FIG. 3 shows how the water uptake capability for air-dried xero-gelsdecreases with increased content of HEC. The decrease in water uptakecapability is partly due to the fact that the resistance to swelling ofthe gel is greater at a higher degree of crosslinking. By mixing CMCNawith HEC it is possible to increase the gel strength of the crosslinkedgel, since HEC has a positive effect on the formation of intermolecularcrosslinks. When the HEC content is below 0.25, the gel strength of thecrosslinked gel is too low for most practical applications.

A further explanation of the reduction in liquid uptake capability withincreasing HEC content may be that the amount of fixed ionic chargespresent on the macromolecular chains is decreased when the HEC contentis increased.

The curves shown in FIG. 4 indicate that drying by extraction withacetone produces a significantly improved liquid uptake capability, aslong as the HEC content does not exceed approximately 50% of the polymermixture.

Example 3

The liquid uptake capability for hydrogels dried using different methodswas compared when the absorbed liquid was a solution of NaCl in water.The ionic strength of the solution was 0.15 mol/l.

It is clear from FIG. 5 that acetone-dried hydrogel has a considerablyhigher uptake capability or swelling capacity than hydrogel which hasbeen dried under vacuum or in air. The improved liquid uptake capabilityfor acetone-dried hydrogel remains, as is evident from FIG. 5, even ifthe DVS concentration is changed.

From FIG. 6, it can be deduced that acetone-dried hydrogel exhibits ahigher liquid uptake capability in synthetic urine when compared toair-dried or vacuum-dried hydrogel, regardless of the ratio between theamount of CMCNa and the amount of HEC.

In the tests presented in FIG. 7 and FIG. 8, synthetic urine (SUR) wasused instead of the NaCl solution used in FIG. 5 and FIG. 6.

From FIG. 7, it is evident that the liquid retention capacity ofacetone-dried hydrogel is higher than for gels which have been dried inother ways. Accordingly, the ability to retain liquid uponcentrifugation of the hydrogels is higher for the acetone-dried gel thanfor hydrogels which have been dried in air or under vacuum, both inabsolute numbers and in relation to the liquid uptake capability of thegels at free swelling.

In FIG. 8, it is shown that the portion of the synthetic urine which isextracted by centrifugation of a gel which has been allowed to swellfreely in synthetic urine is smallest for acetone-dried gel and almostthree times greater for air-dried gel.

Example 4

In FIG. 9, it is shown how desiccation with different solvents affectsthe free swelling capacity of a crosslinked gel based on pectin, and howthe relation in the mixture between the gel and the solvent affects theswelling capacity of the dried gel. The gel was crosslinked with anagent which could not react with alcohols, which means that when usingsolvents such as ethanol and isopropanol, no reaction between thealcohols and the crosslinking agent occured.

The swelling capacity for an air-dried, pectin-based gel is shown as areference. The measurements were carried out by leaving the samples toswell freely in a test tube as described in the second of the freeswelling methods.

As is apparent from FIG. 9, the swelling capacity for a gel which hasbeen dried using isopropanol is better than for a gel which has beendried using ethanol or acetone. All solvent-dried gels exhibit a higherswelling capacity than an air-dried gel.

From the figure, it can further be seen that the relation between theamount of gel and the amount of solvent which is used in the dryingprocess is important for the swelling capacity of the gel. Hence, theswelling capacity is higher for those gels in which a larger quantity ofsolvent was used, since, by using a larger quantity of solvent in thedrying process, the water may be more fully extracted from the gel.

Example 5

In order to investigate how the polymer concentration in the startingreaction solution affects the final properties of the produced hydrogel,mixtures of CMCNa, HEC and DVS were prepared. The CMCNa:HEC ratio was3:1 and the ratio (CMCNa+HEC): DVS was 4:1.

CMCNa, HEC and DVS were dissolved in distilled water at various polymer(CMCNa+HEC) concentrations, namely 3%, 2.5%, 2.3%, 2%, 1.7%, 1.5%, and1%.

After crosslinking and desiccation in acetone, the dry polymer wasimmersed in distilled water until equilibrium was reached. The effect ofthe polymer concentration was assessed by measuring the swellingproperties of the samples. The liquid swollen samples were weighed(using a Mettler AE 100 microbalance with an accuracy of ±10⁻⁴ grams)and the results are found in FIG. 10. Accordingly, FIG. 10 shows theequilibrium swelling ratio in distilled water as a function of thepolymer concentration of CMCNa+HEC in the starting reation solution.

Remarkable differences in water sorption capability were found among theinvestigated samples. However, the water uptake capability for thesample which was prepared from a 1% polymer solution could not bemeasured, since the gel strength of the resulting gel was insufficient.

As can be learned from FIG. 10, the equilibrium water content decreasesas the polymer concentration in the starting solution increases. Thiseffect is due to an increase in the network's elastic response toswelling. When the polymer concentration increases, the averagemolecular weight between two crosslinking points decreases. Accordingly,in order to obtain a gel having good absorption properties, the polymerconcentration in the starting reaction solution should be between1.5%-2.5%. The best compromise between mechanical properties andswelling capacity is obtained with a reaction solution having a polymerconcentration of about 2%.

Example 6

In order to determine the effect on the water uptake capability of theratio “weight of gel”:“weight of acetone” during the dessication step,gels were prepared and dried in different amounts of acetone.

The gels had a CMCNa:HEC weight ratio of 3:1 and the DVS concentrationwas 0.04 mol/l. A water mixture containing 2% by weight of polymer(CMCNa+HEC) was prepared. Potassium hydroxide was used as a catalyst.

The polymer mixture was injected into spherical molds of differentdimensions, where crosslinking took place. Subsequently, gel sphereswere immersed in distilled water until equilibrium water absorption wasreached. The hydrogel samples were desiccated using differentgel/acetone weight ratios. The samples were finally completelydesiccated in vacuum.

The desiccated spheres were then immersed in distilled water untilequilibration. The swelling ratio was evaluated using an electronicmicrobalance (Mettler AE100) with an accuracy of ±104 grams.

The results of the measurements are found in Table 1 and in FIG. 11. Theswelling capacity was found to increase dramatically if the ratiogel/acetone was decreased below approximately 0.06. Accordingly, theamount of acetone used per gram gel should be at least about 16.

TABLE 1 swelling ratio (g of water/g of dry weight ratio polymer) (g ofgel/g of acetone) 35.760 1.272 62.425 0.636 65.185 0.318 77.316 0.15983.636 0.0795 329.76 0.0397 390.98 0.00636

DESCRIPTION OF FIGURES

As can be gleaned from the examples, the absorption capacity of the gelsis highly dependent on the drying method which has been used. Thus, boththe swelling capacity and the liquid uptake capability is lowest forair-dried gel, somewhat higher for gel which has been vacuum-dried andhighest for gel which has been dried by extraction with acetone. Thesedifferences can probably be explained by the fact that different dryingmethods give the gels different morphological properties.

Accordingly, in FIGS. 12-14 there is shown the structure of a gelproduced by crosslinking a 3:1-mixture of CMCNa and HEC and which hassubsequently been dried in different ways. It is thereby evident fromFIG. 12 that a gel which has been air-dried obtains a dense, compactbulk structure. The gel shown in FIG. 13 has a structure with a certaindegree of microporosity, while the gel shown in FIG. 14 and which hasbeen subjected to drying by extraction with acetone has a structureexhibiting a plurality of micropores.

The invention shall not be regarded as being restricted to the Examplesdescribed herein. Accordingly, several further embodiments areconceivable within the scope of protection of the appended claims.

What is claimed is:
 1. A method for production of an absorbentpolysaccharide-based absorption material, comprising: crosslinking aliquid solution containing a starting material to produce an at leastpartially crosslinked liquid-swollen gel, wherein the starting materialis a crosslinkable polysaccharide-based polymer blend comprising anelectrically charged polysaccharide-based polymer and an electricallyuncharged polysaccharide-based polymer; and drying the at leastpartially crosslinked gel by extraction with a polar solvent.
 2. Themethod according to claim 1, wherein the solution containing thestarting material is an aqueous solution.
 3. The method according toclaim 1, wherein the polar solvent is ethanol, acetone or isopropanol.4. The method according to claim 3, wherein the polar solvent is acetoneand the amount of acetone used per gram of at least partiallycrosslinked gel is at least
 16. 5. The method according to claim 1,wherein crosslinking is carried out by adding a covalent crosslinkingagent to the liquid solution.
 6. The method according to claim 5,wherein the weight ratio between the charged polymer and the unchargedpolymer is between 2:1 and 4:1.
 7. The method according to claim 6,wherein the ratio between the charged polymer and the uncharged polymeris 3:1.
 8. The method according to claim 1, wherein the startingmaterial is a mixture of carboxymethyl cellulose and hydroxyethylcellulose.
 9. The method according to claim 1, wherein the liquidsolution contains from 1.5% to 2.5% by weight of the crosslinkablepolysaccharide-based polymer.
 10. The method according to claim 9,wherein the liquid solution contains 2% by weight of the crosslinkablepolysaccharide-based polymer.
 11. The method according claim 1, whereinthe starting material comprises pectin.